1. Field of the Invention
The present invention relates to a method of repairing turbine blades of the type in which an array of turbine blades, arranged in a rotor for example, has a shroud extending around the outer ends of the blades, each blade being provided with at least one tennon adapted to co-operate with the shroud in a manner such that each blade is secured to the shroud.
Turbine blades of the type described which may be used for example in steam turbines are subjected to a harsh environment which after a while causes wear and damage to the blades.
The outer end of the turbine blade in a rotor assembly is the part which has the greatest linear speed in use and hence is the part that is most prone to damage, in particular erosion. Erosion generally occurs because of contact between the turbine blades and particles carried by the steam and also because of contact with the steam itself.
At the high pressure end of the steam turbine, for example, blades are generally subjected to damage from particles of solid matter carried by the steam due to contact between the blades and the particles.
At the low pressure end of the turbine the steam is at a lower temperature having been cooled during passage through the turbine and water condenses therefrom more readily. The steam is therefore wetter, and the transfer of energy by impact on the turbine blades of the steam is greater at the low pressure end of the turbine than it is at the high pressure end.
It is often the area of the blade attached to the shroud, which attachment usually comprises one or more tennons extending substantially radially outwardly from each blade, which are subjected to most wear. The or each tennon from each blade extends into apertures or recesses in the shroud and is affixed thereto by, for example, peening over of the outer end of the tennon.
Replacements of shrouds in turbines is not an uncommon repair procedure. However, the shroud itself is a relatively simple article which can easily be replaced but wear to the end of the turbine blades, and in particular the tennons, causes much greater problems. When wear is considerable, it is necessary to re-build the tennon.
One established repair method involves the building up of a tennon by a welding process followed by machining to reform the tennon to its correct shape so that it may once again be located in a recess or aperture in the shroud.
Such a method of repair has not been found to be entirely satisfactory since the tennon built up by welding in such a manner is primarily of cast material which has inherrent weaknesses and in use can suffer premature failure and in all events gives rise to an unpredictable life span.
Disclosure of the Prior Art
British patent specification No. 2,091,140B proposes a solution to the above problem of performing a turbine blade tennon with a root part, which root part is subsequently attached to the blade by a welding or brazing operation. The root part is preferably of elongate form and is of a shape so that it may key-in to a performed slot in the blade itself so as to enable proper location of the tennon on the blade.
Whereas such a method of repair does overcome many of the problems of building up worn tennons by welding, the machining of the slot to receive the new tennon has to be very accurate which may considerably increase the time necessary to repair a turbine blade and involve the services of specialised persons.